It is well known in the art to exit existing wellbores which may be vertical or angled from the vertical. Such exit wells may be drilled merely to sidetrack the existing wellbores, or may be used for directional drilling. Such exit wells may be drilled at any angle or direction, predetermined or unknown, from the existing wellbores.
In the conventional art, when the existing wellbore is cased, typically with a steel casing, it is known to remove a section of the casing to allow the drill bit to begin cutting the exit well, or to merely cut a window in the steel casing and use a whipstock to direct the drill bit into the adjacent formation. The use of such whipstocks is well-known in the art, for example, in the following United States patents:                U.S. Pat. No. 5,109,924        U.S. Pat. No. 5,551,509        U.S. Pat. No. 5,647,436        U.S. Pat. No. 4,182,423        U.S. Pat. No. 5,806,596        U.S. Pat. No. 5,771,972        U.S. Pat. No. 5,592,991        U.S. Pat. No. 5,636,692        
Thus it has been conventional in this art to use a whipstock in conjunction with a so-called “window mill”. With such configurations, the whipstock is oriented so that it will determine the direction in which the drill bit is eventually to be run through the window cut by the window mill and thus into the formation into which the exit well is to be drilled.
It is also known in this art to use a section mill but without a whipstock. When using the section mill, the mill is used to cut away an entire section of the casing, sometimes 80 to 100 ft. of the casing string, and then that section of the borehole from which the casing has been cut away is pumped full of cement. Once the cement has hardened, conventional sidetracking or directional drilling techniques can be used which do not depend upon the use of a whipstock. Such sectional mills are conventional and are available from various downhole tool companies. For example, a section mill is available from the Baker Oil Tools Division of Baker Hughes, Inc. located in Houston, Tex., such as their Model “D” Section Mill, Product No. 150-72. Such section mills known in this art typically use knives which are hydraulically operated to extend into and cut through the steel casing.
To the best of Applicant's knowledge, those in this art have neither recognized nor utilized a combination of an exit guide with a section mill.